Accreditation and Quality Assurance (v.12, #11)

Comparability and compatibility of proficiency testing results in schemes with a limited number of participants by Ilya Kuselman; Maria Belli; Stephen L. R. Ellison; Ales Fajgelj; Umberto Sansone; Wolfhard Wegscheider (563-567).
Comparability and compatibility of proficiency testing (PT) results are discussed for schemes with a limited number of participants (less than 20–30) based on the use of reference materials (RMs) as test items. Since PT results are a kind of measurement/analysis/test result, their comparability is a property conditioned by traceability to measurement standards applied in the measurement process. At the same time, metrological traceability of the certified value of the RM (sent to PT participants as test items) is also important, since the PT results are compared with the RM certified value. The RM position in the calibration hierarchy of measurement standards sets the degree of comparability for PT results, which can be assessed in the scheme. However, this assessment is influenced by commutability (adequacy or match) of the matrix RM used for PT and routine samples. Compatibility of PT results is a characteristic of the collective (group) performance of the laboratories participating in PT that can be expressed as closeness of the distribution of the PT results to the distribution of the RM data. Achieving quality-of-measurement/analysis/test results in the framework of the concept “tested once, accepted everywhere” requires both comparability and compatibility of the test results.
Keywords: Proficiency testing; Measurement uncertainty; Traceability; Comparability; Compatibility

Good correspondence between evaluations of the proficiency testing data for water measurements according to different scoring schemes was found as a result of implementation of the measurement quality according to the target uncertainty defined in the Drinking Water Directive. The accuracy of the evaluation is determined by the uncertainty of the assigned value, and this uncertainty should be in correspondence with the target uncertainty. Modification of the zeta-score by the thorough application of the target uncertainty is presented. It is convenient for generalised presentation of the proficiency testing data. The allowed bias is included as a linear term on the measured value scale, as it is treated in uncertainty analysis. The importance of implementation of the target measurement uncertainty in other fields of the routine measurements is indicated.
Keywords: Metrology; Interlaboratory comparisons; Proficiency testing; Target uncertainty

Selenium in electrolytic manganese as a reference material for the quality control of aluminium melts by N. Etxebarria; G. Arana; R. Antolín; G. Borge; T. Posada; J. C. Raposo (575-580).
Electrolytic manganese is an important alloying element for aluminium and steel melts. It is mainly added to melts of aluminium in the holding furnace as tablets or minitablets (compressed compacts of manganese and aluminium powders). Selenium derivates are usually added during the production of electrolytic manganese, so some selenium is present in the alloys produced when electrolytic manganese is added to the aluminium furnace. Since the selenium contents of many alloys are of concern from health and environmental perspectives, their values should be provided. In this work, a laboratory reference material (LRM) based on electrolytic manganese was produced to assure our routine quality control method, where selenium is analysed by hydride generation followed by optical emission spectrometry with inductively coupled plasma (HG–ICP–OES). Therefore, the present paper describes in detail the preparation procedure for and the results from homogeneity and stability studies performed on electrolytic manganese LRM. For this purpose, a commercial electrolytic manganese lot was selected and the main factors involved in the preparation of the material (pretreatment step, homogenization, bottling and storage) were carefully studied and established in order to guarantee the long-term stability of the LRM. The results obtained showed that the LRM developed was a fit-for-purpose material for the quality control of the routine analysis of selenium.
Keywords: Electrolytic manganese; Compacts; Aluminium alloying; Reference material

Calibration frequency for gas proportional counters by Margarita Herranz; Raquel Idoeta; Ainara Mira; Fernando Legarda (581-586).
A laboratory that employs nuclear detectors should have a calibration procedure, and its practice is detailed in the bibliography, but the frequency of that calibration is not settled in any case. The aim of this work is to establish an optimum frequency for efficiency calibration in an alpha/beta gas proportional counter, related with the results' adequate routine controls. The results obtained in the application of this purpose to a real instrument (D200 Canberra) are also presented.
Keywords: Quality control; Efficiency; Gas-flow proportional counter

Aspects of the International Atomic Energy Agency (IAEA) Analytical Quality Control Services (AQCS) for marine environmental studies are discussed, focusing on recent laboratory performance studies (LPS) and the production of reference materials for trace metals and organic compounds in various marine matrices. The IAEA has organized seventeen global interlaboratory studies for a range of organic contaminants. Of note has been the inclusion of numerous polychlorinated biphenyl congeners (PCBs) and some sterols of anthropogenic origin. Concurrently, there have been eleven worldwide intercomparison exercises for trace metals in the marine environment, most of which included methylmercury. Although such interlaboratory studies can help improve performance in individual laboratories and regional laboratory networks, the results reveal that problems remain in the determination of some metals and many organic contaminants.
Keywords: Quality control; Quality assurance; Intercomparison exercises; Reference materials; Trace metals; Organic compounds; PAHs; PCBs

UV–Vis spectrophotometric (UV–Vis) and flame atomic absorption spectrometric (FAAS) analysis for iron determination in a pharmaceutical product were compared in terms of uncertainty budgets. Both methods are selective for iron; no interference due to other components present in the matrix was found. The results for the UV–Vis and FAAS methods were 11.4 ± 0.5 mg and 11.9 ± 0.8 mg, respectively, of iron per tablet (at a 95% confidence level). In both methods, the main uncertainty contribution is that due to the calibration function’s non-linearity (0.32 mg and 0.46 mg or 42% and 31% of the standard uncertainty in the case of UV–Vis and FAAS, respectively). This finding encourages us to take uncertainty due to non-linearity into account in uncertainty estimations, even for highly linear methods like UV–Vis. In the ranking of uncertainty contributors, non-linearity is followed by instrument drift in the FAAS method and the uncertainty of volumetric measurements in UV–Vis. In particular, pipetting contributes about 16% of the uncertainty for UV–Vis spectrometry and 10% for FAAS, which is somewhat larger than that usually assumed. The uncertainty contributions of the two methods are compared and discussed in terms of experimental conditions.
Keywords: Uncertainty; ISO GUM; UV–Vis spectrophotometry; FAAS; Iron

The “Guide to the expression of uncertainty in measurement” (GUM) is an extremely important document. It unifies methods for calculating measurement uncertainty and enables the consistent interpretation and comparison of measurement results, regardless of who obtained these measurements and where they were obtained. Since the document was published in 1995, it has been realised that its recommendations do not properly address an important class of measurements, namely, non-linear indirect measurements. This drawback prompted the initiation of the revision of the GUM in the Working Group 1 of the Joint Committee for Guides in Metrology, which commenced in October 2006. The upcoming revision of the GUM provides the metrological community with an opportunity to improve this important document, in particular, to reflect developments in metrology that have occurred since the first GUM publication in 1995. Thus, a discussion of the directions for this revision is important and timely. By identifying several shortcomings of the GUM and proposing directions for its improvement, we hope this article will contribute to this discussion.
Keywords: Metrology; Measurement; Inaccuracy; Uncertainty; Error